1. Field of the Invention
The present invention relates to substrate holding apparatuses for holding workpieces. In particular, the present invention relates to a substrate holding apparatus used in apparatuses for making semiconductors, liquid crystal substrates, magnetic heads, micromachines and the like. The present invention also relates to an exposure apparatus and a method for making a device using the substrate holding apparatus. Moreover, the present invention relates to a method for cleaning a substrate holding section.
2. Description of the Related Art
The wavelength of exposure light sources is currently shifting from 365 nm of the i-line to 248 nm of the KrF laser. Next generation light sources of a shorter wavelength are also being developed. Laser light having a wavelength that is shorter than the wavelength of the KrF laser significantly damages optical elements, resulting in increased operation cost. Thus, in addition to the trend of light sources towards shorter wavelengths, development of high-resolution exposure apparatuses is directed to high numerical aperture (NA) projection optical systems. The effective depth of focus of the optical system, however, decreases as the NA increases. Thus, in order to achieve a sufficiently large depth of focus, the process tolerances must be maintained as much as possible by suppressing the curvature of field in the optical system and by improving the uniformity of the substrate thickness and the planar precision of a chuck.
In consideration of such circumstances, incidental incorporation of foreign materials between a wafer and the chuck must be avoided in order to assure a practical depth of focus. Various types of foreign materials, such as resists and worn parts formed by sliding of a contact portion of a transfer hand, may be adhered to the rear surface of the wafer during steps prior to the exposure steps. It is difficult to determine (1) where foreign materials are adhered and (2) the sizes of the foreign materials. Further, foreign materials interposed between the wafer and the chuck cause significant effects. For example, when a foreign material having a particle size of 1 .mu.m is interposed, the wafer rises over a region having a diameter of 12 mm from the center of the foreign material, resulting in decreased resolution. If the foreign material remains on the chuck, wafer defects will occur at the same position on many wafers.
In conventional apparatuses, grinders provided for cleaning slide on the surface of the chuck, under pressure, in order to remove the foreign materials adhered to the chuck by directly scraping off the foreign materials. This cleaning process is performed manually or automatically and causes a decrease in throughput due to the apparatus being shut down during the cleaning process.